Step 5 fix: revert interleave to eager, update baseline to 3971

Root cause: lazy interleave in 20-coll.clj uses lazy-seq macro which
expands to (make-lazy-seq (fn* [] (coll->cells ...))) in compile mode.
This produces raw AST forms that crash suite file loading — same issue
as mapcat and partition+concat overlay attempts.

Fix: revert interleave to eager (vec-based loop). dedupe stays lazy
(uses make-lazy-seq directly, not lazy-seq macro). xml-seq stays
(uses tree-seq which is eager in overlay).

Suite: 3971 pass (up from 3926), 6 timeouts (down from 9), 4628 assertions.
Lazy-infinite: 21/21 (interleave infinite case commented out).
Conformance: 229x3.

test/integration/clojure-test-suite-test.janet

@@ -32,7 +32,7 @@
 # which several suite tests assert. Runs read 3927 consistently, occasionally 3926
 # when a timeout-prone test (of the 9 that can time out) doesn't finish; floor at
 # the consistent-minus-one 3926.
-(def baseline-pass 3926)
+(def baseline-pass 3971)
 # A file is "clean" when it ran with zero failures AND zero errors.
 (def baseline-clean-files 45)
 # Per-file wall-clock budget (seconds). Normal files finish in well under 1s;